Electromagnetic wave resonators are essential for generating and amplifying of coherent electromagnetic waves, wavelength selection, highly sensitive detection of an electromagnetic wave due to enhancement of an electromagnetic field, and exhibiting various non-linear effects.
With respect to light such as infrared rays, visible rays and ultraviolet rays, conventional resonators are composed of geometric optical elements and have much larger volumes than those of the wavelengths. A typical example of them is a laser oscillator composed of dielectric multilayer mirrors. In recent years, however, researches and developments have been carried out seeking resonators with a small volume. This is because a quantum electrodynamic effect is expected in resonators as small as the size of wavelengths. The intensity of such effect is indicated by the Purcell factor. The Purcell factor is proportional to a Q value and inversely proportional to a resonator volume. The Q value is an index indicating energy stored in a resonator or degree of electric field enhancement. Arguments based on the factor are applicable to subjects such as change in fluorescence relaxation and also to enhancing effect on Raman scattering due to the electric field enhancement. In order to obtain a large Purcell factor, it is important to confine a strong electromagnetic field to a small volume. It should be noted that the subjects discussed here are not limited to those in a narrow sense referred to as the Purcell effect.
In the region of radio waves, thin sticks of perfect conductor having a length of about half to the wavelength have been known for some time that they function as a type of resonator localizing electromagnetic waves, and they have been used as antennas. In the region of microwaves, confinement to a wave length space by cavity resonators, i.e. high efficiency reflection with perfect conductors, has been used for some time.
In the light range having visible light at the center, wave length resonators were realized only recently.
The first form of them is a dielectric microsphere. An extremely large Q value was achieved due to a low-loss propagating mode circulating around the equator during total reflection, called the whispering gallery mode. However, confinement by a microsphere about the same in size to a wavelength is so weak that the microsphere does not work as a good resonator. Since a sphere with a relatively large diameter from several times to several tens of times the wavelength has to be employed for obtaining a large Q value, the volume cannot be very small.
The second form is a photonic crystal resonator. A photonic crystal is a two or three dimensional dielectric multilayer mirror. A lattice defect intentionally built in an environment where light cannot propagate in any direction due to a periodic structure achieves a condition in which light localizes only in the defect spot, and thus the neighboring area of the defect functions as a resonator. The size of the confined area is, however, reduced to only about Wavelength/(2×Refractive Index).
Surface plasmons are known as a factor in light propagation. Although the term surface plasmon is a designation from a perspective of particle picture, classical electromagnetic plasma waves are the actual subject for arguments. Surface plasmons are caused by a surface wave localized at an interface and exponentially attenuated as it leaves from the interface, and observed in metals in which the real part of each dielectric constant has a negative value in a range from visible light to infrared light. Since a component perpendicular to the interface has an imaginary value among the three components of wave vector, the other two components have larger values than those in free space. That is, the wavelength is shorter than that of a light wave propagating in a space. The wavelength is determined by the dielectric properties as well as the structure of the material, and even shorter wavelengths can be realized.
An example of surface plasmon in metal provided with a slit is reported in the following reference: F. J. Garcia-Vidal et al., “Transmission and focusing of light in one-dimensional periodically nanostructured metals”, Physical Review B, 66, 155412 (2002).